1. Field of the Invention
The present invention relates to systems for performing backscatter absorption gas imaging (BAGI), and more specifically, it relates to a pulsed linescanner for use in BAGI imaging.
2. Description of Related Art
BAGI is an existing and patented technique disclosed in U.S. Pat. No. 4,555,627, titled xe2x80x9cBackscatter absorption gas imaging systemxe2x80x9d. Simply stated, the patent covers the use of infrared laser-illuminated imaging for the remote video visualization of gas plumes. It describes the coupling of an infrared laser to an infrared camera to produce an instrument that views a scene in the infrared as the laser illuminates the scene. The system produces, therefore, a laser-illuminated video picture of the scene. If a gas plume is present that can absorb light at the center wavelength, it creates a shadow in the picture that is essentially a video image of the gas plume. BAGI is currently being commercialized by Laser Imaging Systems (LIS), which offers systems operating in the 9-11 xcexcm wavelength range based on the use of CO2 lasers.
U.S. Pat. No. 3,317,730 discloses a method for determining atmospheric pollution by the detection of backscattered modulated infrared radiation.
U.S. Pat. No. 3,832,548 to Wallack shows a general infrared absorption detector in which infrared radiation first passes through a filter means having a plurality of positions for transmitting selected wavelengths, and then passes through a sample cell to a detector.
U.S. Pat. No. 4,204,121 to Milly shows a mobile detector comprising a vertical sampling array for quantifying emission rates from pollution sources.
U.S. Pat. No. 4,264,209 to Brewster shows a system for producing an indication of a concentration of a gas of interest in which the gas is illuminated and the output is filtered alternately with two filters, one at an absorption band of a gas to be detected, the other at a passband outside the absorption band.
U.S. Pat. No. 4,262,199 to Bridges, et al., shows a mobile infrared target detection and recognition system including an assembly of infrared detection elements which scan a field of view to produce a signal representative of the infrared level from point to point.
U.S. Pat. No. 3,829,694 to Goto discloses apparatus for detecting gases or particles using Mie scattering of pulsed light beams to detect resonance absorption.
U.S. Pat. No. 3,517,190 to Astheimer discloses a method for monitoring stack effluent from a remote position by illuminating the effluent across a broad spectral band and detecting the reflected illumination in two spectral regions: one in an absorption band and one outside the absorption band to determine the quantity of absorbing gas from the signal ratio.
The publication Kulp et al., xe2x80x9cDevelopment of a pulsed backscatter-absorption gas-imaging system and its application to the visualization of natural gas leaksxe2x80x9d, Appl. Opt. 37 3912-3922 (1998), describes the development of a pulsed BAGI imager that uses full-field illumination at a laser pulse repetition rate of 30 Hz.
The publication Powers et al. xe2x80x9cDemonstration of differential backscatter absorption gas imagingxe2x80x9d, Appl. Opt. 39 1440-1448 (2000) described the development of a pulsed BAGI imager that uses full-field illumination at a laser repetition rate of 30 Hz and is capable of differential detection. It operates in a way that is adversely affected by system motion.
The publication of Imeshev et al. xe2x80x9cLateral patterning of nonlinear frequency conversion with transversely varying quasi-phase-matching gratingsxe2x80x9d Optics Letters 23 673-675 (1998) describes the use of periodically poled lithium niobate with lateral patterning to produce second harmonic frequency output beam with a flat-topped spatial profile.
It is an object of the present invention to provide a pulsed linescanner for use in a backscatter absorption gas imaging (BAGI) system.
It is another object of the invention to provide a BAGI imager that is capable of operation with pulsed laser sources. The term xe2x80x9claserxe2x80x9d is intended to include lasers as well as any other light sources with spectral and brightness properties meeting the requirements presented in this teaching. For example such lightsources could include a laser followed by a frequency conversion device or an incandescent beam from a gas discharge source.
It is another object of the invention to provide a pulsed linescanner that is capable of differential imaging.
It is another object of the invention to provide methods for acquisition of images by a pulsed linescanner in ways that are immune to moderate camera motion (such as might be encountered in hand-held or vehicle-mounted operation).
It is another object of the invention to provide methods for acquisition of images by a pulsed differential linescanner in ways that are immune to moderate camera motion (such as might be encountered in hand-held or vehicle-mounted operation).
It is another object of the invention to provide means for achieving a linescanning BAGI imager that is capable of both single-wavelength and differential imaging.
It is another object of the invention to provide a pulsed linescanned imager that by concentrating the transmitted light in a small number of rows achieves a higher backscattered signal from a given target using a given laser pulse energy and repetition rate than can be obtained by a system employing full-field illumination.
These and other objects of the invention will be apparent to those skilled in the art based on the teachings herein.
The present invention is an active (laser illuminated) imaging system that is suitable for use as a BAGI imager. As in all BAGI systems, the present invention employs a laser, tuned to a wavelength absorbed by the gas to be detected, that is coupled to a suitable video camera. The laser illuminates the scene as the camera images it. Gases present in the imaged scene are visualized when they absorb the laser light, thus creating a dark region in the video picture. This allows the imager to be used to rapidly detect and pinpoint leaks of gases (such as hydrocarbons found in leaks at petroleum refineries or in natural gas pipelines) that absorb light produced by the laser employed. To maximize the attenuation, the spectral profile of the laser must be narrower than the target gas absorption linewidth and must be centered at the peak of the strongest absorption line that is not affected by interfering species. Operation away from the peak of the gas absorption or with lasers having a broader spectral width than the absorption feature is also possible with an associated reduction in detection sensitivity.
The invention described here uses a pulsed laser as its illumination source and creates images by linescanning. Plume visualization is accomplished in either of two modesxe2x80x94termed single-wavelength imaging and differential imaging. In single-wavelength imaging the scene is illuminated only with laser radiation having a wavelength absorbed by the gas. The gas image is produced when the gas plume attenuates the backscatter return from solid objects in the imaged scene. In differential imaging, the scene is illuminated by radiation at two different wavelengths; one strongly absorbed by the gas, termed the xe2x80x9con-wavelengthxe2x80x9d, and one that is not absorbed (or weakly absorbed), termed the xe2x80x9coff-wavelengthxe2x80x9d. For every displayed frame, a backscatter signal is collected from each scene pixel at each wavelength. An image generated from the on-wavelength backscatter would be identical to the previously described single-wavelength image. An image generated from the off-wavelength backscatter would, on the other hand, contain no gas image (or only a weak gas image). In differential imaging both the on-and off-wavelength signals are processed to generate a differential image, in which the differences between the two frames are emphasized. An example of such processing is the log-ratio, where the logarithm of the ratio of the on-wavelength signal to the off-wavelength signal is displayed. The two wavelengths are selected to be close enough together that the reflectivities of the target scene surfaces are nearly identical at each wavelength. Thus, ideally, the on-wavelength and off-wavelength return signals differ from one another only in regions of the scene that are occluded by the gas plume. Ideally, differential processing by the log-ratio approach will produce an image containing only the gas plume image and no elements of the scene image. This will allow the plume image to be displayed at high contrast, which eases its visual recognition. Additionally, quantitative measurements of path-integrated gas concentration can be made following a calibration of the differential absorption signal. It is also within the skill of the art to overlay the differential-mode image of the gas plume on a passive video image of the scene to assist in identifying the location of the gas plume.
The present invention differs from that described in the original BAGI patent (U.S. Pat. No. 4,555,627). The system description in the original patent specified use of a cw laser emitting light of a single frequency whose beam was raster-scanned across the target. Thus, it was only useful for single-wavelength imaging and could not achieve the additional sensitivity provided by differential imaging. Also, the requirement for a cw beam restricts the choice of laser that can be used for gas imaging, as some operate only in a pulsed mode. This is often true for lasers that use nonlinear mixing to generate light of a particular output frequency because the efficiency of the nonlinear mixing process scales with the peak power of the light field used to pump the nonlinear crystal. Pulsed lasers can create output pulses of high peak power. Thus, a BAGI approach employing a pulsed format offers additional choice of lasers to use for gas imaging and, thus, more choice of wavelengths and other relevant attributes (compactness, portability, ruggedness, efficiency). This is important because there are no lasers currently available in pulsed or cw formats that are suitable for BAGI and are tunable over all wavelengths of interest for gas detection. The ability to operate in a pulsed mode allows operation in some important wavelength ranges (e.g., tunable operation in the 8-12 xcexcm range, which coincides with the molecular fingerprint region) in which there are no broadly-tunable cw lasers available.
The present invention differs from previous disclosures of pulsed BAGI imaging systems and differential pulsed BAGI imaging systems. In the publication Kulp et al. discussed above, a pulsed BAGI imager is described that uses a 30-Hz-repetition-rate-laser to illuminate the full field-of-view of a two-dimensional infrared focal-plane array camera. In the publication Powers et al. described above, that approach is extended to allow differential imaging in a manner in which the collection of on-wavelength frames is interleaved with that of off-wavelength framesxe2x80x94thus an on-wavelength frame is collected {fraction (1/30)}th of a second after an off-wavelength frame. Pairs of on-wavelength and off-wavelength frames are processed to generate log-ratio differential image frames at a rate of 15 Hz. The time between on- and off-wavelength frames is long relative to the timescale of movement of the camera, objects in the scene, or of the plume itself. Thus, a difficulty that occurs in practical use of the published system is misregistration between the on- and off-wavelength pixels, leading to significant errors in the differential image. These limitations can preclude operation in certain important modes in which movement occurs, such as when the system is handheld by an operator (such as for roaming inspection by an operator in a refinery), or when it is operated from a moving vehicle such as a van (for leak detection from natural gas distribution lines in residential areas) or from a helicopter (for leak detection from natural gas transmission lines in rural areas).
Operation in a linescanned mode is also consistent with the use of high repetition-rate pulsed lasers. xe2x80x9cHigh repetition ratexe2x80x9d for the laser is defined as being a rate higher than the image frame rate, which is typically between 10 to 60 Hz. The system described here uses a laser having a several kHz repetition rate, which is a relatively common for diode-pumped solid-state pump lasers. Diode pumped solid-state lasers can have advantageous features including small size and power consumption, and the present invention makes it possible for gas imaging to employ these lasers.